How to edit second skin of mesh created using solidify
The deer in your tutorial was topologically a sphere with one hole in it, so only one rim between inside and outside had to be found, and ripped.

Your gun has more holes than that; if you cut one rim, there are more left connecting the inside and outside.

A possible method:


Before applying the modifier, while editing edges, in the Select Menu in the header, go for 'All by Trait' > 'Non Manifold'. This will select all the holes in your mesh before solidification.
Apply the Modifier without changing that selection
After applying the modifier, in Edit Mode, V rip the still selected (and duplicated) edges.


This will separate your object into 3. Inside, Outside, and Rim.

You will note that the modifier has thickened some areas which are already shells with thickness, (e.g. around the barrel) and so some of the  'inside' surfaces are outside some of the 'outside' surfaces. This is quite confusing, and unprintable. I guess you're going to remove those, and seal up the holes.